Publisher Summary This chapter aims to demonstrate how to decode letters sent from the deep interior of the earth, based on the analyses of morphological features recorded in mineral crystals. For this purpose, diamond crystals are selected as a representative example of crystal growth taking place in deep-seated magma and in the subduction zone, beryl crystals occurring in pegmatites that represent crystallization in super-critical liquid phase at the latest stage of magmatic solidification, and trapiche ruby formed by contact metasomatism due to intrusion of a granitic magma into carbonate rocks. Gem minerals are selected as they occur as nearly perfect single crystals and information obtained on such crystals serves as very useful diagnostic features in distinguishing natural from synthetic gemstones. The chapter summarizes the present understanding of morphological aspects of crystals and presents discussions on diamond, beryl, and trapiche ruby. In terms of morphological features of crystals, the chapter includes external morphology, morphology of polyhedral crystals, surface microtopography of crystal faces, growth sectors, and growth banding that demonstrate how growth, dissolution, or transformation of natural crystals proceed.